One of the persistent challenges faced in the development of semiconductor technology is the desire to increase the density of circuit elements and interconnections on substrates without introducing spurious interactions between them. Unwanted interactions are typically prevented by providing gaps or trenches that are filled with electrically insulative material to isolate the elements both physically and electrically. As circuit densities increase, however, the widths of these gaps decrease, increase their aspect ratios and making it progressively more difficult to fill the gaps without leaving voids. The formation of voids when the gap is not filled completely is undesirable because they may adversely affect operation of the completed device, such as by trapping impurities within the insulative material.
Common techniques that are used in such gapfill applications are chemical-vapor deposition (“CVD”) techniques. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. Plasma-enhanced CVD (“PECVD”) techniques promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes when compared with conventional thermal CVD processes. These advantages may be further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive. While each of these techniques falls broadly under the umbrella of “CVD techniques,” each of them has characteristic properties that make them more or less suitable for certain specific applications.
In some instances where gaps have a large aspect ratio and narrow width, gaps have been filled with thermal CVD techniques using a “dep/etch/dep” process by sequentially depositing material, etching some of it back, and depositing additional material. The etching step acts to reshape the partially filled gap, opening it so that more material can be deposited before it closes up and leaves an interior void. Such dep/etch/dep processes have also been used with PECVD techniques, but some thermal and PECVD techniques are still unable to fill gaps having very large aspect ratios even by cycling deposition and etching steps. More recently, dep/etch/dep processes have additionally be used with HDP-CVD techniques, even though such technique have generally had superior gapfill capabilities because they provide a sputtering component to the deposition simultaneous with film growth. It is for this reason, in fact, that HDP-CVD techniques are sometimes referred to as simultaneous dep/etch processes.
With all such dep/etch/dep techniques, a remaining difficulty has been an inability to control the etching characteristics as carefully as desired during the etching phase. Lack of sufficient control of the etching characteristics may result in such undesirable results as corner clipping, in which a structural portion defining the gaps is damaged because of excessive etching at that point. Furthermore, less control over the etching characteristics also decreases the efficiency of the overall gapfill process since reopening the gap during the etching phase may not provide as useful a profile as might be achieved with greater control. These issues have become increasingly important as integrated circuit designs have increasingly employed smaller minimum feature sizes, now often less than 0.1 μm.